The intervertebral disc functions to stabilize the spine and to distribute forces between vertebral bodies. A normal disc includes a gelatinous nucleus pulposus, an annulus fibrosis and two vertebral end plates. The nucleus pulposus is surrounded and confined by the annulus fibrosis.
It is known that intervertebral discs are prone to injury and degeneration. For example, herniated discs are common, and typically occur when normal wear, or exceptional strain, causes a disc to rupture. Degenerative disc disease typically results from the normal aging process, in which the tissue gradually looses its natural water and elasticity, causing the degenerated disc to shrink and possibly rupture.
Intervertebral disc injuries and degeneration are frequently treated by replacing or augmenting the existing disc material. Current intervertebral disc replacement procedures tend to utilize synthetic materials such as polyethylene mesh to encapsulate a central core of hydrogel. These synthetic materials are woven into textured fabrics whose rough surfaces may accelerate wear of the encapsulated hydrogel or the bone endplates of the intervertebral body. Such wear may generate wear particles, and can cause adverse biological responses such as osteolysis in the vertebral body endplate bone and subsequent subsidence of the implant.
For example, reports on the use of prosthetic nucleus replacement devices with polyethylene mesh jackets have indicated subsidence of these devices into the endplates of the vertebral bodies. Subsidence is also due to the rigid compliance of the jacket and hard hydrogel core. This modulus mismatch with the vertebral bone, combined with the other design features mentioned above, contributes to implant subsidence.
In addition to intervertebral discs and joints, other synovial joints are present in the mammalian appendicular skeleton. A typical synovial joint comprises two bone ends covered by layer of articular cartilage. The cartilage is smooth and resilient, and facilitates low-friction movement of the bones in the joint.
The bone ends and associated cartilage are surrounded by a joint capsule—a “sack” of membrane that produces synovial fluid. The capsule and fluid protect and support the cartilage and connective tissue, carrying nutrients to the articular cartilage and removing the metabolic wastes.
The articular cartilage is a thin (2-3 mm) layer of hyaline cartilage on the epiphysis of the bone. It lacks a perichondrium, and thus has a limited capacity for repair when damaged. Additionally, the natural aging process can cause the articular cartilage to degenerate somewhat, reducing its capacity to protect and cushion the bone ends.
Zygapophysial joints, better known as facet joints, are the mechanism by which each vertebra of the spine connects to the vertebra above and/or below it. Each joint comprises two facet bones—an inferior facet and a superior facet—with the inferior facet of one vertebra connecting to the superior facet of an adjacent vertebra. The joints facilitate movement of the vertebra relative to each other, and allow the spine to bend and twist.
As in all synovial joints, where the facets contact each other there is a lining of cartilage lubricated by a thin layer of synovial fluid. The cartilage and synovial fluid decrease friction at the joint, extending joint life and preventing inflammation and associated pain.
As the natural aging process progresses, the cartilage covering the joint may deteriorate and start to fray. The fraying process may cause pieces of cartilage to break free, and the previously smooth surfaces may become rough. The facet bones then begin to rub together, creating friction which leads to further deterioration of the joint. Moreover, the nerves associated with the joint become irritated and inflamed, causing severe pain and restricting movement of the spine.
Techniques for addressing degeneration of synovial joints in general, and facet joints in particular, joint have heretofore relied primarily on injections to block pain and reduce inflammation. This treatment is only temporary though, and rarely leads to any significant improvement of the underlying condition.
It can be seen from the above that a need exists for vertebral disc implants that avoid the problems associated with the use of synthetic materials in augmenting, repairing or replacing all or part of an intervertebral disc. It can also be seen that a need exists for materials and methods effective for treating degenerating synovial joints, and particularly for materials and methods effective for supplementing or replacing the cartilage that lubricates and protects the joint. The present invention addresses those needs.